Such apparatuses and methods are known from the prior art. The process begins with the production of plastic preforms by means of an injection molding machine. These plastic preforms normally comprise a body as well as a thread provided on said body. This thread is normally already provided in its final shape, whereas the body is expanded, by means of an expansion process, into the plastic container to be produced. It is common practice that the plastic preforms are heated by means of a heating unit prior to the actual expansion process.
More recently, it has become more and more common practice to produce the plastic material, which is used for forming the preforms, from recycled material, or to use recycled material. The use of recycled resins entails the problem that, due to batch variations, stretch-blow molding manufacturing processes necessitate greater efforts to operate the machines in question. In other words, the plastic material used for producing the preforms is not precisely uniform and, consequently, the plastic preforms produced from said plastic material are not precisely uniform either. Such batch variations originate from the different raw materials which are used in recycling processes and which are also beyond the sphere of influence of the producer of the recyclate.
These different mixtures lead to different and thus also varying heat absorption characteristics of the preforms.
These variations lead, in turn, to variations of the preform temperature achieved on the heating path of the blow molding machine, i.e., in the final analysis, an intervention on the part of the operators will be necessary more frequently; these operators have to very well trained. It may also happen that whole batches are lost, since they are not shaped in a sufficiently uniform manner, e.g. because they were not sufficiently heated.